1. Field of the Invention
The present invention relates to a method of forming a diamond crystal which has superior characteristics as electronic materials.
2. Description of the Related Art
Diamonds have various superior characteristics which cannot be obtained by other materials, e.g., a large band gap (5.5 eV), large carrier mobility (electron; 1800 cm.sup.2 /V.multidot.S, hole; 1600 cm.sup.2 /V.multidot.S), large thermal conductivity [200 W/(m.multidot.K)], high hardness, and high wear resistance.
Therefore, methods of synthesizing diamonds from the vapor phase, particularly, a chemical vapor deposition process, have been studied recently. Primary methods of forming diamonds are as follows.
1) A thermal filament CVD process in is which material gas decomposed by heating a filament made of, e.g., tungsten to about 2000.degree. C. so that a diamond crystal is deposited on a substrate. PA0 2) A microwave plasma CVD process in which material gas is decomposed by microwave plasma (usually at 2.45 GHz) so that a diamond crystal is deposited on a substrate. PA0 3) An RF plasma CVD process in which material gas is decomposed by RF Plasma (usually at 13.56 MHz) so that a diamond crystal is deposited on a substrate. PA0 4) A burning flame process in which a diamond crystal is formed by using an oxygen--acetylene flame, PA0 5) An ECR plasma CVD process in which material gas is decomposed by forming ECR (Electron Cyclotron Resonance) discharge in combination with a microwave and a magnetic field so that a diamond crystal is formed on a substrate. PA0 6) A thermal plasma CVD process in which material gas is decomposed by DC or high-frequency thermal plasma under a Pressure near the atmospheric Pressure so that a diamond crystal is formed. PA0 7) Of late, Japanese Patent Laid-Open No. 5-32489 proposes a plasma CVD process using plasma in the ultrashort wave range of 30 to 300 MHz.
However, the prior art methods of forming a diamond crystal described above have problems as discussed below.
The thermal filament CVD process of 1) is advantageous in that a diamond crystal can be formed in a large area and at a relatively high forming rate, but is disadvantageous in that the filament is carbonized or deteriorates over time and reproducibility in forming diamonds is low.
The microwave plasma CVD process of 2) is advantageous in that the plasma density is high, the forming rate of a diamond crystal is relatively high, and the reproducibility is good, but is disadvantageous in that plasma tends to localize, making it difficult to form a diamond crystal in a large area.
The RF plasma CVD process of 3) is advantageous in that a diamond crystal can be formed in a large area and the reproducibility is good, but is disadvantageous in that the plasma density is low and the forming rate of a diamond crystal is low.
The burning flame process of 4) is advantageous in that the forming rate of a diamond crystal is very high, but is disadvantageous in that the reproducibility is low and a diamond crystal is difficult to develop in a large area.
The ECR plasma CVD process of 5) is advantageous in that a diamond crystal can be formed in a large area and the reproducibility is good, but is disadvantageous in that the forming rate of a diamond crystal is slow because the pressure is required to be kept low (generally less than 100 Pa) in order to create the ECR discharge.
The thermal plasma CVD process of 6) is advantageous in that the forming rate of a diamond crystal is very high and the reproducibility is good, but is disadvantageous in that it is difficult to form a diamond crystal in a large area.
The plasma CVD process in the ultrashort wave range (30 to 300 MHz) of 7) is superior to any of the above-described processes, but a diamond crystal cannot be always stably formed on a substrate of a large area due to unevenness of the plasma. Also, because a diamond crystal under formation is subjected to impacts of ions having energy of several tens eV from the plasma, such ion impacts may reduce the forming rate of a diamond crystal and deteriorate crystallinity. Thus, a further improvement in the crystal growing rate and crystallinity is demanded.